1. Field of the Invention
The present invention relates to a display device using a thin film transistor at least for a pixel portion.
2. Description of the Related Art
In recent years, a technique that is used to form thin film transistors using semiconductor thin films (with thicknesses of from several nanometers to several hundreds of nanometers, approximately) formed over substrates having an insulating surface has been attracting attention. Thin film transistors are applied to a wide range of electronic devices such as ICs or electro-optical devices, and prompt development of thin film transistors that are to be used as switching elements in image display devices, in particular, is being pushed.
As a switching element in an image display device, a thin film transistor using an amorphous semiconductor film, a thin film transistor using a polycrystalline semiconductor film, or the like is used. As a method for forming a polycrystalline semiconductor film, there is known a technique in which a pulsed excimer laser beam is shaped into a linear laser beam by an optical system and an amorphous silicon film is scanned and irradiated with the linear beam so as to crystallize the amorphous silicon film.
As a switching element in an image display device, further, a thin film transistor using a microcrystalline semiconductor film is used (see Reference 1: Japanese Published Patent Application No. H4-242724; and Reference 2: Japanese Published Patent Application No. 2005-49832).